This invention relates to an improved fuel injection system and fuel injector which effectively controls the flow rate of fuel injected into the combustion chamber of an engine.
In most fuel supply systems applicable to internal combustion engines, fuel injectors are used to direct fuel pulses into the engine combustion chamber. A commonly used injector is a closed-needle injector which includes a needle assembly having a spring-biased needle valve element positioned adjacent the needle orifices for resisting blow back of exhaust gas into the pumping or metering chamber of the injector while allowing fuel to be injected into the cylinder. The needle valve element also functions to provide a deliberate, abrupt end to fuel injection thereby preventing a secondary injection which causes unburned hydrocarbons in the exhaust. The needle valve is positioned in a needle cavity and biased by a needle spring to block fuel flow through the needle orifices. In many fuel systems, when the pressure of the fuel within the needle cavity exceeds the biasing force of the needle spring, the needle valve element moves outwardly to allow fuel to pass through the needle orifices, thus marking the beginning of injection. In another type of system, such as disclosed in U.S. Pat. No. 5,676,114 to Tarr et al., the beginning of injection is controlled by a servo-controlled needle valve element. The assembly includes a control volume positioned adjacent an outer end of the needle valve element, a drain circuit for draining fuel from the control volume to a low pressure drain, and an injection control valve positioned along the drain circuit for controlling the flow of fuel through the drain circuit so as to cause the movement of the needle valve element between open and closed positions. Opening of the injection control valve causes a reduction in the fuel pressure in the control volume resulting in a pressure differential which forces the needle valve open, and closing of the injection control valve causes an increase in the control volume pressure and closing of the needle valve. U.S. Pat. No. 5,463,996 issued to Maley et al. discloses a similar servo-controlled needle valve injector.
Internal combustion engine designers have increasingly come to realize that substantially improved fuel supply systems are required in order to meet the ever increasing governmental and regulatory requirements of emissions abatement and increased fuel economy. It is well known that the level of emissions generated by the diesel fuel combustion process can be reduced by decreasing the volume of fuel injected during the initial stage of an injection event while permitting a subsequent unrestricted injection flow rate. As a result, many proposals have been made to provide injection rate control devices in closed needle fuel injector systems. One method of controlling the initial rate of fuel injection is to spill a portion of the fuel to be injected during the injection event. For example, U.S. Pat. No. 5,647,536 to Yen et al. discloses a closed needle injector which includes a spill circuit formed in the needle valve element for spilling injection fuel during the initial portion of an injection event to decrease the quantity of fuel injected during this initial period thus controlling the rate of fuel injection. A subsequent unrestricted injection flow rate is achieved when the needle valve moves into a position blocking the spill flow causing a dramatic increase in the fuel pressure in the needle cavity. However, the needle valve is not servo-controlled and, thus, this needle assembly does not include a control volume for controlling the opening and closing of the needle valve and the timing of injection at least primarily fuel pressure dependent.
Another manner of optimizing combustion is to create pilot and/or post injection events. Most current diesel injectors include fixed needle orifice areas sized to provide optimum injection duration at rated speed and load with the highest allowable injection pressure. However, in order to optimize combustion, pilot and post injection events must include extremely small quantities of fuel at high injection pressures. With a fixed spray orifice size, this results in an extremely short event that is difficult to control. To compensate, the needle opening velocity may be reduced so that the fuel flow is throttled before the spray orifices during the pilot and post injection events. However, needle velocity is not easily controllable from injector to injector, while throttling wastes fuel energy and does not provide optimum combustion performance. At low speed and light load, it is also desirable to have small spray orifices to increase injection duration without lowering injection pressure.
Another fuel injector design providing some limited control over fuel injection rate and quantity includes two needle valve elements for controlling the flow of fuel through respective sets of injection orifices. For example, U.S. Pat. No. 5,458,292 to Hapeman discloses a fuel injector with inner and outer injector needle valves biased to close respective sets of spray holes and operable to open at different fuel pressures. The inner needle valve is reciprocally mounted in a central bore formed in the outer needle valve. However, the opening of each needle valve is controlled solely by injection fuel pressure acting on the needle valve in the opening direction such that the valves necessarily open when the injection fuel pressure reaches a predetermined level. Consequently, the overall and relative timing of opening of the valves, and the rate of opening of the valves, cannot be controlled independently. Moreover, the valve opening timing and rate is dependent on the injection fuel pressure.
U.K. Patent Application No. 2266559 to Hlousek discloses a closed needle injector assembly including a hollow needle valve for cooperating with one valve seat formed on an injector body to provide a main injection through all the injector orifices and an inner valve needle reciprocally mounted in the hollow needle for creating a pre-injection through a few of the injector orifices. However, the valve seat allowing the inner valve needle to block the pre-injection flow is formed on the hollow valve member and the inner valve needle is biased outwardly away from the injector orifices. This arrangement requires a third valve seat for cooperation with the inner valve element when in a pre-injection open position to prevent flow through all of the injector orifices, resulting in an unnecessarily complex and expensive assembly. Also, this assembly is designed for use with two different sources of fuel requiring additional delivery passages in the injector. In addition, like Hapeman, this design requires the timing and rate of opening of at least one of the needle valves to be controlled by fuel injection pressure thereby limiting injection control.
U.S. Pat. No. 5,199,398 to Nylund discloses a fuel injection valve arrangement for injecting two different types of fuels into an engine which includes inner and outer poppet type needle valves. During each injection event, the inner needle valve opens a first set of orifices to provide a preinjection and the outer needle valve opens a second set of orifices to provide a subsequent main injection. The outer poppet valve is a cylindrical sleeve positioned around a stationary valve housing containing the inner poppet valve.
U.S. Pat. No. 5,899,389 to Pataki et al. discloses a fuel injector assembly including two biased valve elements controlling respective orifices for sequential operation during an injection event. A single control volume may be provided at the outer ends of the elements for receiving biasing fluid to create biasing forces on the elements for opposing the fuel pressure opening forces. However, the control volume functions in the same manner as biasing springs to place continuous biasing forces on the valve elements. As a result, the needle valve elements only lift when the supply fuel pressure in the needle cavity is increased in preparation of a fuel injection event to create pressure forces greater than the closing forces imparted by the control volume pressure.
U.S. Pat. Nos. 4,202,500 to Keiczek and 4,215,821 to Eblen both disclose injectors having two sets of injector orifices controlled by respective needle valves.
Although some systems discussed hereinabove create different stages of injection, further improvement is desirable. Therefore, there is need for a servo-controlled fuel injector for providing enhanced control over injection timing and flow rate, especially in variable supply pressure systems.
It is an object of the present invention, therefore, to overcome the disadvantages of the prior art and to provide a fuel injector which is capable of effectively and predictably controlling the rate of fuel injection.
It is another object of the present invention to provide a servo-controlled injector capable of effectively providing a dual injection so as to minimize emissions.
It is another object of the present invention to provide a fuel injection system capable of providing a variable supply pressure and selectively providing either a low fuel injection rate followed by a high fuel injection rate or only a single low fuel injection rate.
It is yet another object of the present invention to provide a fuel injection system capable of selectively producing a wide variety of injection flow rates or rate shapes depending on engine operating conditions while remaining compatible with existing fuel systems.
It is a further object of the present invention to provide an injector for use in a variety of variable pressure fuel systems, including common rail system and accumulator pump systems, which effectively controls the rate of injection at each cylinder location.
Still another object of the present invention is to provide an injector which is capable of selectively creating different injection rate shapes to optimize emissions and fuel economy.
Yet another object of the present invention is to provide an injector which is compatible with existing pilot activated fuel injection mechanisms and methodologies.
Another object of the present invention is to provide an injector which permits injection duration to be extended and fueling accuracy improved at part load conditions.
These and other objects of the present invention are achieved by providing a fuel injection system for injecting fuel into the combustion chamber of an engine comprising a variable pressure fuel supply for supplying fuel at various pressure levels and a fuel injector including an injector body containing an injector cavity and a plurality of injector orifices communicating with one end of the injector cavity to discharge fuel into the combustion chamber. The fuel injector also includes a plurality of injector orifices including a first set of orifices and a second set of orifices. The injector body also includes a fuel transfer circuit for supplying fuel to the injector orifices and a first needle valve element positioned in the injector cavity for controlling fuel flow through the first set of orifices. A first valve seat is formed on the injector body and the first needle valve element is movable from a closed position against the first valve seat blocking flow through the first set of injector orifices to an open position permitting flow through the first set of injector orifices. The fuel injector further includes a second needle valve element positioned in the injector cavity for controlling fuel flow through the second set of injector orifices. The fuel injector also includes a second valve seat formed on the injector body wherein the second valve element is movable from a closed position against the second valve seat blocking flow through the second set of injector orifices to an open position permitting flow through the second set of injector orifices. The fuel injector further includes a first control volume positioned adjacent an upper end of the first needle valve element for receiving fuel and a second control volume positioned adjacent an upper end of the second needle valve element for receiving fuel. The fuel injector also includes a drain circuit for draining fuel from the first and the second control volumes to a low pressure drain. The injector also includes an injection control valve positioned along the drain circuit for controlling the flow of fuel from the first and the second control volumes through the drain circuit to permit movement of the first and the second needle valve elements between the open and the closed positions. The first needle valve element may be telescopingly received within a cavity formed in the second needle valve element to form a sliding fit with an inner surface of the second needle valve element. A throttle passage may be provided in the second needle valve element to restrict fuel flow upstream of the first set of injector orifices during a fuel injection event. A first biasing spring may be provided for biasing the first needle valve element toward the closed position and a second biasing spring for biasing the second needle valve element toward the closed position. The first biasing spring may be positioned within the cavity of the second needle valve element. The first and the second biasing springs may be positioned in overlapping relationship. The injector may also include a separator positioned between the first and the second control volumes and biased into a position by the first biasing spring. The injector may also include a first control volume charge circuit including a charge groove formed in an inner surface of the separator.
The present invention is also directed to the above described fuel injection system wherein the first or inner needle valve element is adapted to open at a predetermined low pressure level while the second needle valve element remains in the closed position. In addition, the second or outer needle valve element is adapted to move into an open position at a higher fuel supply pressure greater than the predetermined low pressure level. The present invention is also directed to a fuel injection system and fuel injector wherein the outer needle valve element is adapted to open at a low pressure level while the inner needle valve element remains in the closed position until a higher predetermined supply pressure level is supplied.